Liquefaction of gases typically employs cryogenic processes comprised of refrigeration cycles generated by expanding at least one refrigerant. A variety of refrigerants might be employed, for example, a mixed refrigerant (MR) stream having a mixture of nitrogen, methane, ethane/ethylene, propane, butanes and pentanes is a commonly used refrigerant in many base-load liquefied natural gas (LNG) plants. The refrigeration cycles employed for liquefaction might be a single mixed refrigerant cycle (SMR), propane-precooled mixed refrigerant cycle (C3MR), dual mixed refrigerant cycle (DMR), mixed refrigerant-expander hybrid cycles such as AP-X™, nitrogen or methane expander cycles, cascade cycle or any other appropriate refrigeration process. The composition of the MR stream is typically optimized for the feed gas composition and operating conditions.
In SMR systems, the SMR is typically compressed in at least two stages of compression and cooled back to close-to-ambient temperature. Some liquid is typically formed at the outlet of at least one intercooler of the SMR system since the mixture contains heavy components to provide refrigeration in the pre-cooling stage. However, such intermediate pressure liquid being formed during compression can lead to irreversible mixing of streams of different temperature, pressure, and/or composition, which can reduce the efficiency of the liquefication operation. Current attempts to eliminate this problem introduce additional cost, complexities and equipment, possibly reducing reliability of the system.
Therefore, there is a need for an improved gas liquefaction process that reduces or eliminates the intermediate pressure liquid being formed while maintaining high efficiency and reliability of the process while also maintaining a low equipment count.